Delta Variant in California: What Vaccinated People Should Know About Breakthrough Cases, Symptoms and Travel

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A customer takes advantage of outdoor dining in San Francisco's North Beach neighborhood on Dec. 4, 2020. (Beth LaBerge/KQED)

The highly contagious delta variant is gaining ground in the United States and is now expected to become the dominant COVID-19 strain in California.

It's led the Los Angeles County Department of Public Health to "strongly recommended" that residents once more start wearing masks inside indoor spaces like restaurants and stores — even if they're vaccinated against the coronavirus. California's mask mandate for vaccinated people was previously dropped when the state reopened on June 15, with some exceptions for spaces like public transit, schools and health care settings.

It's hard not to be unsettled by the news of the delta variant's spread. But how worried should people be in the Bay Area, where COVID cases are relatively low and vaccination rates are high?

KQED spoke to Dr. Kirsten Bibbins-Domingo, chair of UCSF's Department of Epidemiology and Biostatistics, about the relative risks of the delta variant and how much vaccinated people in the Bay Area should be considering modifying their behavior. Because the updates on the delta variant "can be a little bit scary, and a little bit confusing," says Bibbins-Domingo, it's important to know the relative risks at play.


This interview has been edited for length and clarity.

Will the Bay Area Follow L.A. and Recommend Masking Again?

Certainly the vaccines are highly effective. They are not 100% perfect. And I think for each of this is just understanding our own level of tolerance, knowing that the variants are on the rise, and are more transmissible.

There are people who will in that setting want to be masking more often. But I don't think yet in the Bay Area, given that our case rates are low, that there will actually be a public health order saying that we should do that.

I don't think we want to create the confusion by saying, "Oh, everyone needs to mask up again." But I do think there is the reality that many people in the Bay Area still are wearing their masks, even though they've been vaccinated.

And I think that it's up to your own personal level of of risk-taking in an environment where the variants are present. There are many people who are going to choose to wear their masks, particularly indoors, when in a public setting, when they don't know whether the people around them are vaccinated or not.

How Worried Should I Be About Breakthrough Cases Among Vaccinated People? How Rare Are They Really?

I can't really overstate what great news it is that particularly the mRNA vaccines, those made by Pfizer and Moderna, are 90% effective against this delta variant.

It's even better news to realize that even when there are breakthroughs, that they really have not been the ones that are causing very severe disease that are landing you in the hospital or dying, except in very rare cases. So even when they're breakthrough infections, people tend to have more mild disease.

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Listeners may know that the World Health Organization has also suggested that people who are vaccinated should wear masks indoors because of the rise of the delta variant. But that reflects also that worldwide, many, many more people are not vaccinated or are vaccinated with a type of vaccine that is not as effective.

[Breakthrough infections] are rare because these vaccines are effective. Ironically, though, when you look at a population where a large number of people are vaccinated, then even in rare cases of a highly transmissible variant, breaking through means that you start to have larger numbers of people. And so it looks like there's a problem with the vaccines. There really isn't a problem with the vaccines, but they aren't 100%.

And I think that is why many people in this environment might decide that if they're in a public indoor setting when they don't know whether people around them are vaccinated, that they will take that extra layer of protection of wearing a mask. But everyone should understand that the vaccines on their own are in the 90% range of effectiveness against this delta variant, and that even when there are breakthroughs, they usually aren't of the type that are landing people in the hospital with severe illness.

What Delta Variant Symptoms Should I Watch for, Even if I'm Vaccinated?

It's the same set of symptoms that people have experienced for a while. The sore throat, the things that might be cold or flu-like symptoms. And it is the recommendation that if you experience those symptoms, even if you've been vaccinated, that you should be tested for this reason.

Again, it's far less common — the vaccines are highly effective. In an environment where a delta variant starts to take hold, they will be cases mostly in the unvaccinated people. That is certainly what we're seeing in California and around the country. But the breakthrough cases among the much larger groups of vaccinated people will also mean that some of them will have symptoms, and will want to be tested.

A sign urges people to get tested for a COVID-19 variant in Blackburn, England. The U.K. is experiencing a surge in the delta variant, which was first identified in India.
A pedestrian walks past an electronic information board displaying COVID-19 information relating to a 'variant of concern in the area' in Blackburn, northwest England, on June 16, 2021. The U.K. government on Monday announced a four-week delay to the full lifting of coronavirus restrictions in England due to a surge in infections caused by delta, which first appeared in India. (Oli Scarff/AFP via Getty Images)

Should the Delta Variant Change My Behavior if I'm Traveling Outside of the Bay Area?

One has to recognize that in the Bay Area we have both high vaccination rates and a high rate of people still masking. And both of those things are contributors to the very low case rates we have in the Bay Area.

That's not true even in other parts of the state, frankly. So I think when you are in those settings, it is prudent to be cautious. If you're vaccinated and outdoors, those are really very safe settings. But if I were in an environment that had lower rates of vaccination, I would certainly — in an indoor setting for myself personally — make sure that I have have a mask on.

How at Risk From the Delta Variant Are Unvaccinated Kids?

This is a highly transmissible variant that is transmissible also in children, and they make up a large fraction of the unvaccinated population in most places. And it is the reason that even though each one of us who is vaccinated might feel personally protected, we're living in an environment of unvaccinated people that include large groups of people like children and preteens who definitely can be infected.

And that is why families with children who have not yet been vaccinated are [still] wearing masks this summer. And why all of us have to be vigilant, to watch the transmission of this variant.

How Does 'Long COVID' Play Into All This?

This is one of those areas where the science is still evolving, and we are realizing more and more how important this is.

We've seen the figures of the acute presentations of COVID, and the deaths from COVID. But really, the longer-term sequelae [after effect] of COVID is important, and is affecting a larger number of people, I think, than we previously had really appreciated. And I think it is one more reason why we really have to make sure that everyone is protected from that initial infection.

Because even if your risk of having a complication acutely is low — as we know it is, for example, in young adults — we also know that COVID can do much more than what it does in the acute setting. It's yet another reason, frankly, to be vaccinated.

Vaccination is the key to controlling this. And so the most important thing for all of us to think about is how we can encourage as many people as possible to actually get these vaccines that are highly effective.